The present invention relates to a wire-extension end detector for a wire-cut electric discharge machine, which is highly reliable in detecting action.
An apparatus provided in a wire-cut electric discharge machine for carrying out an automatic extension of a wire, disconnected intentionally to allow a workpiece to be shifted to change a discharge machining portion thereof or disconnected accidentally during the discharge machining, is disclosed in Japanese Patent Disclosure No. 62-44319, for example. In this type of automatic wire extension apparatus, a wire, supplied through a wire passage of an upper wire guide arranged above a workpiece, is fed into a groove or hole of the workpiece while being restricted by a machining liquid jetted from a nozzle formed in the upper wire guide. The wire is then passed through a wire passage of a lower wire guide. Subsequently, the wire is fed to a conveyor device arranged under the lower wire guide. The conveyor device includes a pair of belts pressed against each other, so that the wire is conveyed, while held between the belts, in between a wire drive roller and a pinch roller arranged downstream of the coveyor device. The wire extension is completed when the wire is held bewteen the wire drive roller and the pinch roller. When the wire extension comes to an end, an electric power supplying section which is electrically connected to the wire becomes connected through the wire, the electrically conductive outer peripheral wall of the wire drive roller, and an electrically conductive brush pressed against the outer wall of the wire drive roller, to an electric power supplied section which is connected to the brush. The resulting supply of electric power from the power supplying section to the power supplied section is detected by a suitable means as an end of wire extension.
As described above, the prior art wire-extension end detector uses the brush pressed against the wire drive roller, and therefore is liable to suffer disconnection between the brush and the roller as the brush is worn away with repeated use, thus failing to detect the end of a wire extension.